As discussed in the last edition of TechTalk (issue number 77), materials perform differently when they operate at various stresses, temperatures, speeds, and in corrosive environments. For this reason it is important to select the right fan impeller material for the application. In this edition we will look at specific applications and why some materials are more suitable than others.
Car Park Exhaust Fans
National Construction Code - NCC 2013 Table E2.2a Class 7a Buildings states “Fans with metal blades suitable for operation at normal temperature may be used”
The intent of this clause is that while car park fans are not classed as an essential service, it is recognized that it is more beneficial for exhaust fans to run in the event of a fire to assist in clearing smoke, rather than stopping. Therefore steel and aluminum impellers are typically used for car park exhaust fans.
Hazardous Environments
When fans are being used in a hazardous environment where explosive gases, vapours, dusts or fibres may be present there is a need to reduce potential ignition sources that may cause the hazardous substance to explode or catch fire.
Ignition sources can occur between components rotating relative to each other. There are four main ways this can occur:
- Sparks between steel and/or iron components striking each other. This is the most likely cause of sparks so the fan design and construction must eliminate this possibility.
- A “Thermit” reaction caused when aluminium and rusty or oxidised steel strike each other.
- Sparks from a build-up of static electricity on non-conductive parts, such as plastic fan blades or housings, which can spark to an earthed component.
- Heat from two surfaces brushing against each other.
Fan impeller to casing/housing tip clearances are normally increased to reduce this possibility.
If choosing an impeller made from aluminium material, there must be sufficient clearance to avoid contact with the steel housing. As only “allowable material pairings” may be used, an anti-spark ring or inlet cone made of aluminium, brass or copper is normally supplied for hazardous environment fans with aluminium blades.
A Glass Reinforced Plastic impeller can potentially generate a spark through the build-up of static electricity and subsequent discharge to an earthed component. Therefore, an anti-static impeller material should normally be used in this instance as they have a much lower surface resistivity to avoid the static electricity build-up.
Smoke Spill Fans
While impeller types are not specified, Australian Standards AS1668.1 1998 requires that:
4.8.1" …The smoke-spill fan shall be type-tested for these rating requirements in accordance with AS4429(Int).”
As the rating requirements are 200°C for 2 hours in sprinklered buildings, or 300°C for 30 mins in un-sprinklered buildings, steel or aluminium impellers are normally the only types to operate at such high temperatures.
Kitchen Exhaust Fans
There is a common misconception that kitchen exhaust fans require steel or aluminium fan impellers. However AS1668.1 1998 11.2.5 Amdt 1 states that:
“Kitchen exhaust fan casings and cowls shall be manufactured from non-combustible materials that have a fusing temperature above 1000°C.”
The clause is specifically targeting the fan housing so that in the event of a fire the integrity of the housing is maintained to restrict fire spreading to other areas. Impellers are not required to be non-combustible and therefore plastic and composite impellers may be used in kitchen exhaust fans.
Refer to your local Fantech office or agent for assistance and advice on fan materials and applications.
